1. Field of the Invention
Embodiments of the present invention are directed in general to a two-phase, silicate-based photoluminescent material. More specifically, the present embodiments are directed to yellow-emitting phosphors comprising two silicate-based phases, the phosphors having applications in high brightness white LED illumination systems, LCD display panels, plasma display panels, and yellow illumination systems.
2. Description of the Related Art
White LEDs are known in the art, and they are relatively recent innovations. It was not until LEDs emitting in the blue/ultraviolet region of the electromagnetic spectrum were developed that it became possible to fabricate a white light illumination source based on an LED. Economically, white LEDs have the potential to replace incandescent light sources (light bulbs), particularly as production costs fall and the technology develops further. In particular, the potential of a white light LED is believed to be superior to that of an incandescent bulb in lifetime, robustness, and efficiency. For example, white light illumination sources based on LEDs are expected to meet industry standards for operation lifetimes of 100,000 hours, and efficiencies of 80 to 90 percent. High brightness LEDs have already made a substantial impact on such areas of society as traffic light signals, replacing incandescent bulbs, and so it is not surprising that they will soon provide generalized lighting requirements in homes and businesses, as well as other everyday applications.
There are several general approaches to making a white light illumination system based on light emitting phosphors. To date, most white LED commercial products are fabricated based on the approach shown in FIG. 1A, where light from a radiation source contributes to the color output of the white light illumination. Referring to the system 10 of FIG. 1A, a radiation source 11 (which may be an LED) emits light 12, 15 in the visible portion of the electromagnetic spectrum. Light 12 and 15 is the same light, but is shown as two separate beams for illustrative purposes. A portion of the light emitted from radiation source 11, light 12, excites a phosphor 13, which is a photoluminescent material capable of emitting light 14 after absorbing energy from the source 11. The light 14 can be a substantially monochromatic color in the yellow region of the spectrum, or it can be a combination of green and red, green and yellow, or yellow and red, etc. Radiation source 11 also emits blue light in the visible that is not absorbed by the phosphor 13; this is the visible blue light 15 shown in FIG. 1A. The visible blue light 15 mixes with the yellow light 14 to provide the desired white illumination 16 shown in the figure.
Alternatively, a newer approach has been to use non-visible radiation sources that emit light in the ultra-violet (UV). This concept is illustrated generally at reference numeral 20 in FIG. 1B, which illustrates an illumination system comprising a radiation source that emits in the non-visible such that the light coming from the radiation source does not contribute substantially to the light produced by the illumination system. Referring to FIG. 1B, substantially non-visible light is emitted from radiation source 21 as light 22, 23. Light 22 has the same characteristics as light 23, but the two different reference numerals have been used to illustrate the following point: light 22 may be used to excite a phosphor, such as phosphor 24 or 25, generating photoemitted light 26 and 27, respectively, but the light 23 emitted from the radiation source 21 which does not impinge on a phosphor does not contribute to the color output 28 from the phosphor(s) because light 23 is substantially invisible to the human eye.
What is needed is an improvement over the silicate-based, yellow phosphors of the prior art where the improvement is manifested at least in part by an equal or greater conversion efficiency of the blue excitation light to the desired yellow output from the phosphor. It is also desired for the phosphor to have temperature stability up to 200° C., and an enhanced color rendering. Furthermore, it is critical for the improvement of manufacturing yield to have a yellow phosphor which has a broad and consistent excitation efficiency in a wavelength range from about 400 nm to about 480 nm. The desired yellow phosphor with over all high performance and low cost may be used in conjunction with a blue LED to generate high brightness white light whose color output is stable with temperature, and whose color mixing results in the desired uniform color temperature and color rendering index.